1. Field of the Invention
The present invention relates to a radar system for observing weather phenomena and a radar signal processing method therefor, and particularly relates to a technique for performing a radar range evaluation based on a weather model set corresponding to weather conditions and for optimizing radar system parameters with high efficiency.
2. Background Art
A transmitting frequency of a radar system for observing weather phenomena, which is principally intended for observations of a rainfall region, has heretofore been less than an X band (this system will hereinafter be called a "low-frequency radar system").
However, the use of a radar system having a higher transmitting frequency (hereinafter called a "high-frequency radar system"), which is intended for observations of clouds or fog including particles smaller than rain droplets, has increased in recent years.
A radar system using a millimeter wave whose transmitting wavelength is on the order of millimeter, is able to observe cloud particles and fog particles each being on the order of a several tens of .mu.m in particle diameter. One such radar system has been disclosed in A. L. Pazmany et al. "An Airborne 95 GHz Dual-polarized Radar for Cloud Studies", IEEE Trans. Geosci. Remote Sensing, vol. 32, no. 4, pp. 731-739, (July. 1994).
Since a radio wave is greatly attenuated due to the atmosphere and rainfall particles as compared with the low-frequency radar when targets such as rainfall or clouds are observed by the high-frequency radar disclosed in the above-described publication, the radar range is reduced.
Since the attenuation or the attenuation factor of the radio wave depends on an existing region of rainfall or clouds and weather conditions such as the amount of water droplets therein, etc., the radar range of the radar system for observing them greatly depends on weather conditions and observing directions. In order to increase or extend the radar range in the observing direction in which the radar range is short because the radio wave is greatly attenuated, a number of integration in the observing direction must be increased. It is also necessary to perform antenna scanning as to be able to obtain an increase in the number of integration in the observing direction. Now consider the method to obtain a Doppler velocity in order to measure the movement of a target such as a cloud or the like. Since the Doppler frequency is proportional to a transmitting frequency, it is necessary to increase a pulse repetition frequency (hereinafter called "PRF") with a view toward setting the maximum Doppler velocity measurable by the high-frequency radar system to a magnitude of a predetermined value or more.
The execution of a coherent radar process, such as the measurement of the Doppler velocity, must obtain a plurality of received pulse signals within a correlation time of a weather echo, and it is known that the correlation time of the weather echo is proportional to inverse of the transmitting frequency. As a result, a high PRF is needed to measure the Doppler velocity by the high-frequency radar system.
Since, however, the increase in PRF causes ambiguity in the distance measurement, a problem arises in that reflected echoes from beyond the maximum observed distance determined according to PRF, i.e., multiple echoes are mixed into a received signal. Since sources of the multiple echoes and radio waves respectively vary in reflectivity and attenuation depending on weather conditions, the influence of the multiple echoes also depends on the weather conditions and the observing direction.
Thus, the decrease in radar range due to the attenuation of the radio wave and the ambiguity in distance measurement vary depending on the weather conditions. However, the conventional radar system for observing the weather phenomena is accompanied by a problem that the radar range evaluation is not performed corresponding to the weather conditions, and hence the radar observations are not always carried out based on the suitable radar system parameters corresponding to the weather conditions.